Detection of Hot Gas in the Filament Connecting the Clusters of Galaxies

A&A 482, L29–L33 (2008) Astronomy DOI: 10.1051/0004-6361:200809599 & c ESO 2008 Astrophysics

Letter to the Editor

Detection of hot gas in the ﬁlament connecting the clusters of galaxies Abell 222 and Abell 223

N. Werner1, A. Finoguenov2,J.S.Kaastra1,3, A. Simionescu2, J. P. Dietrich4,J.Vink3, and H. Böhringer2

1 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: [email protected] 2 Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany 3 Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands 4 ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany Received 18 February 2008 / Accepted 17 March 2008 ABSTRACT

Context. About half of the baryons in the local Universe are invisible and – according to simulations – their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 105 < T < 107 K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations. Aims. We aim to probe the low gas densities expected in the large-scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (z = 0.21), which has a favorable orientation approximately along our line-of-sight. This filament has been previously detected using weak lensing data and as an over-density of colour-selected galaxies. Methods. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton. Results. We present observational evidence of X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet-decomposed soft-band (0.5–2.0 keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≈1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT = 0.91±0.25 keV, and its emission measure corresponds to a baryon density of (3.4±1.3)×10−5(l/15 Mpc)−1/2 cm−3, where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of ρ/ ρC≈150. The properties of the gas in the filament are consistent with results of simulations of the densest and hottest parts of the warm-hot intergalactic medium. Key words. cosmology: large-scale structure of Universe – X-rays: galaxies: clusters – galaxies: clusters: individual: Abell 222 – galaxies: clusters: individual: Abell 223

1. Introduction distant quasars was reported in the vicinity of the Virgo and Coma clusters and interpreted as indication of the presence of According to the standard theory of structure formation, the dense and hot WHIM concentrations near the clusters (Fujimoto spatial distribution of matter in the Universe evolved from small et al. 2004; Takei et al. 2007). A blind search for WHIM was perturbations in the primordial density field into a complex performed by Nicastro et al. (2005), who reported the discov- structure of sheets and filaments with clusters of galaxies at the ery of two intervening systems at z > 0 in absorption along intersections of this filamentary structure. The filaments have the line-of-sight to the blazar Mrk 421 in the Chandra Low been identified in optical surveys of galaxies (e.g. Joeveer et al. Energy Transmission Grating Spectrometer (LETGS) spectra. 1978; Baugh et al. 2004; Tegmark et al. 2004), but the dominant However, Kaastra et al. (2006) carefully reanalysed the LETGS fraction of their baryons is probably in the form of a low density spectra, confirmed the apparent strength of several features, warm-hot gas emitting predominantly soft X-rays. but showed that their detection was not statistically significant. The existence of this warm-hot intergalactic medium Furthermore, the results reported by Nicastro et al. (2005)were (WHIM) permeating the cosmic-web is predicted by numerical neither confirmed by later Chandra LETGS observations nor by hydrodynamic simulations (Cen & Ostriker 1999; Davé et al. the XMM-Newton RGS data of Mrk 421 (Kaastra et al. 2006; 2001), which show that in the present epoch (z 1–2) about Rasmussen et al. 2007). The detection of the WHIM in absorp- 30% to 40% of the total baryonic matter resides in the filaments tion using current instrumentation will remain difficult, because connecting clusters of galaxies. These simulations predict that the expected absorption features are weak. In the large searched the WHIM spans a temperature range of 105 < T < 107 Kanda redshift space to distant blazars, statistical fluctuations with a density range of 4 × 10−6 cm−3 to 10−4 cm−3, which corresponds strength comparable to the strength of absorption lines expected to 15–400 times the mean baryonic density of the Universe. Gas from the WHIM will therefore always appear. A search for the at these temperatures and densities is difficult to detect (Davé WHIM from the densest and hottest parts of the large-scale et al. 2001). structure filaments with current instruments is more realistic in Several attempts have been made to detect the WHIM both emission than in absorption. in absorption and emission, but none have succeeded in pro- Searches in emission with ROSAT and XMM-Newton viding an unambiguous detection. Marginal detection of O viii have found several candidates for WHIM filaments (Kull & and Ne ix absorption features in XMM-Newton RGS spectra of Böhringer 1999; Zappacosta et al. 2002; Finoguenov et al. 2003;

Article published by EDP Sciences L30 N. Werner et al.: Detection of hot gas in the ﬁlament connecting the clusters of galaxies Abell 222 and Abell 223

Kaastra et al. 2003). However, the association of the detected Table 1. The CXB components determined in a region free of clus- soft emission with the WHIM has always been difficult and un- ters emission outside of the virial radii of the clusters. The unabsorbed certain. In some cases, there is still a possible confusion with the fluxes are determined in the 0.3–10.0 keV band. f indicates parameters Galactic foreground emission, or with the Solar-wind charge ex- kept fixed during spectral fitting. vii change emission that also produces O lines and soft emission Comp. kT/Γ Flux in excess of cluster emission. (keV / phot. ind.) (10−12 erg cm−2 s−1 deg−2) Since the X-ray emissivity scales with the square of the gas LHB kT = 0.08 f 3.4 ± 1.1 density, X-ray emission studies are most sensitive to the dens- GH kT = 0.17 ± 0.03 2.9 ± 0.9 est gas concentrations of the WHIM that are located, accord- EPL Γ=1.41 f 22 f ing to simulations, near clusters of galaxies. The most appro- PL Γ=0.78 ± 0.16 49 ± 7 priate method to study the physical properties of the WHIM is therefore to target distant binary clusters in which the connecting filament bridge is aligned approximately along our line- details see Snowden et al. 2008). To eliminate the flux pollu- of-sight. A filament between cluster pairs observed with such tion from the faint point sources due to the large wings of the a favourable geometry has a higher surface brightness than fil- point-spread function (PSF) of XMM-Newton, we applied an aments observed at a different orientation angle. By observing image-restoration technique (Finoguenov et al., in prep) that the variation of the surface brightness in X-ray images extracted uses a symmetric model for the PSF of XMM-Newton cali- in the low energy band, the presence of a WHIM filament can brated by Ghizzardi (2001). The flux from the extended wings be revealed as a bridge between the clusters. This is a differen- of the PSF of sources detected on small scales (sum of the 8 tial test with respect to the background and it is less sensitive to and 16 scale) was estimated by performing a scale-wise wavelet systematic uncertainties. analysis (Vikhlinin et al. 1998). This estimated flux was used to The most promising target for detecting the warm-hot gas in subtract the PSF model prediction from the larger scales and to a filament is Abell 222/223, a close pair of massive clusters of accordingly increase the flux on the smaller scales. The uncer- galaxies at redshift z ≈ 0.21, separated by ∼14 on the sky, which tainties in the point-source subtraction were added to the total corresponds to a projected distance of ∼2.8 Mpc. Assuming that error budget to reduce the significance of the residuals. both clusters are part of the Hubble flow without peculiar ve- ff ∆ = . ± . locities, the observed redshift di erence of z 0 005 0 001 2.2. Spectral analysis (Dietrich et al. 2002) translates to a physical separation along the line of sight of 15±3Mpc.Dietrich et al. (2005) reported indica- We extracted the filament spectrum from a circular extraction re- tions of a filament connecting both clusters using weak lensing, gion with a radius of 1.6, located between the clusters, and cen- optical, and X-ray (ROSAT) data. They found an X-ray bridge tred at a projected distance of 5.76 (1.19 Mpc) from both clus- with a linear extent of about 1.3 Mpc in projection, connecting ter cores. We determined the background spectrum using two the clusters at a 5σ significance level, and an over-density of independent sets of regions to provide a consistency check. We galaxies in the filament at a 7σ significance level. In the weak used, firstly, a circular region of 3.2 radius, located at a pro- lensing image, the filament was present at a 2σ significance. jected distance larger than 1.6 Mpc from both clusters, which is We present results of a new, deep (144 ks) XMM-Newton beyond their virial radii determined using weak lensing analysis observation of the bridge connecting the two clusters. We con- (r200 = 1.28 ± 0.11 Mpc for A 222 and r200 = 1.55 ± 0.15 Mpc firm its presence by both imaging and spectroscopy, and we es- for A 223; Dietrich et al. 2005). Secondly, we extracted two timate the temperature, density, entropy, and the total mass of other circles of 1.6 radii located on the same central chip of the the gas in the filament. Throughout the paper we use H0 = EPIC/MOS detectors. Both circles are located at the same pro- −1 −1 70 km s Mpc , ΩM = 0.3, ΩΛ = 0.7, which imply a linear jected distance to one of the clusters as the extraction region cen- scale of 206 kpc arcmin−1 at the redshift of z = 0.21. Unless tred on the filament. Using the same chip and the same projected specified otherwise, all errors are at the 68% confidence level distance to the clusters for determining both the source and the for one interesting parameter (∆χ2 = 1). background spectra ensures the best possible subtraction of the instrumental and X-ray background components. Point sources were excluded from the spectral extraction regions. For the iden- 2. Observations and data analysis tification of point sources, we also used archival Chandra data (45.7 ks). The regions used for spectral extraction, for both the The pair of clusters Abell 222/223 was observed with filament and the background, are indicated in Fig. 1. XMM-Newton in two very nearby pointings centred on the fila- We subtracted the instrumental background from all spec- ment on June 18 and 22, 2007 (revolutions 1378 and 1380) with tra using closed-filter observations. Since the instrumental back- a total exposure time of 144 ks. The calibrated event files were ground level changes from observation to observation and in- produced using the 7.1.0 version of the XMM-Newton Science creases with time over the years, we normalised the closed-filter Analysis System (SAS). After removing the time intervals af- data, for each extraction region, by the 10–12 keV count rate for fected by high particle-background, we obtained 62.6 ks of good EPIC/MOS data and by the 10–14 keV count rate for EPIC/pn / / time for EPIC MOS and 34.4 ks for EPIC pn. data. We then modelled the spectra for each background region set with the components described below, and used these mod- 2.1. Image analysis els to subtract the cosmic X-ray background from the filament spectrum. We coadded the final 0.5–2.0 keV and 2.0–7.5 keV images from We analysed the spectra using the SPEX package (Kaastra all instruments and both pointings. The background was sub- et al. 1996) in the 0.35 to 7 keV band for EPIC/MOS and tracted and the images were corrected for vignetting. The chip 0.6–7 keV band for EPIC/pn, ignoring the 1.3–1.8 keV band be- number 4 in EPIC/MOS1 was excluded from the analysis because of the presence of strong instrumental lines. We fixed the cause of the anomalously high flux in the soft band (for more Galactic absorption in our model to the value determined from N. Werner et al.: Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223 L31

Fig. 2. The spectrum of the filament between the clusters A 222/223 – the data points on the top were obtained by EPIC/pn and below by Fig. 1. Wavelet-decomposed 0.5–2.0 keV image of Abell 222 (to the EPIC/MOS. The contributions from the X-ray background and from South) and Abell 223 (the two X-ray peaks to the North). We show the filament to the total model are shown separately. only sources with >5σ detection, but for these sources we follow the emission down to 3σ. The filament connecting the two massive clusters is clearly visible in the image. The regions used to extract the filament spectrum and to determine the background parameters are indicated by red and yellow circles, respectively. To verify our result, we created an image modelling the X-ray emission from the two clusters without a filament with two beta models determined by fitting the surface-brightness profile − of A 222 and A 223. We then applied the wavelet-decomposition H i radio data (N = 1.6 × 1020 cm 2, Kalberla et al. 2005). We H algorithm to this image, which did not reveal a bridge such as modelled the background with four components: the extragalac- that observed between A 222 and A 223. tic power-law (EPL), to account for the integrated emission of unresolved point sources (assuming a photon index Γ=1.41 If the emission in the bridge between the clusters originated and a 0.3–10.0 keV flux of 2.2 × 10−11 erg s−1 cm−2 deg−2 af- in a group of galaxies within the filament, then this group would ter extraction of point sources, De Luca & Molendi 2004); two be well resolved in the image. Groups of galaxies at the redshift of this cluster emit on spatial scales of ≈0.5 as shown in thermal components, to account for the local hot bubble (LHB) σ emission (kT = 0.08 keV) and for the Galactic halo (GH) emis- Fig. 1 by the six extended sources detected at the 4 significance 1 around Abell 222/223 marked by ellipses numbered from 1 to 6. sion (kT2 ∼ 0.2 keV); and an additional power-law to account for the contamination from the residual soft proton particle back- The filament seen in the image is about 6 wide, which at the ground. The best-fitted fluxes, temperatures, and photon indices redshift of the cluster corresponds to about 1.2 Mpc. of the background components, using the 3.2 circular extraction The observed filament is about an order of magnitude fainter region, are shown in Table 1. than the ROSAT PSPC detected filament reported by Dietrich We modelled the spectrum of the filament with a et al. (2005). The ROSAT detection was based on an image σ = . collisionally-ionized plasma model (MEKAL), the metallicity of smoothed by a Gaussian with 1 75 , and the filament could which was set to 0.2 Solar (proto-Solar abundances by Lodders be the result of the combination of emission from the cluster 2003), which is the lowest value found in the outskirts of clus- outskirts and from a few previously unresolved point-sources be- ters and groups (Fujita et al. 2008; Buote et al. 2004). However, tween the clusters. since the average metallicity of the WHIM is unknown, we re- The point-sources (mostly AGNs) detected by port results also for metallicities of 0.0, 0.1, and 0.3 Solar. The XMM-Newton and Chandra do not show an overdensity free parameters in the fitted model are the temperature and the between the clusters, and the uniform contribution from the emission measure of the plasma. distant unresolved AGNs was subtracted from our image. Our detection limit for point sources is ∼0.8 × 10−15 erg s−1 cm−2, which corresponds to a luminosity of 1041 erg s−1 for the faintest 3. Results resolved galaxy at the cluster redshift. The number of X-ray emitting galaxies with this luminosity required to account for 3.1. Imaging the observed emission in the filament would be ∼10 times larger than expected from the distribution function of Hasinger (1998), We detect the filament in the wavelet-decomposed soft-band ρ/ ρ∼ (0.5–2.0 keV) X-ray image with 5σ significance. In Fig. 1, assuming a filament over-density of 100. we show the wavelet-decomposed image produced by setting σ the detection threshold to 5 and following the emission down 3.2. Spectroscopy to 3σ (for details of the wavelet decomposition technique see Vikhlinin et al. 1998). There is a clear bridge in the soft emis- The clusters of galaxies Abell 222 and Abell 223 are bright and sion between the clusters, which originates in an extended com- relatively hot. The temperature of A 222, extracted from a region ponent. We note that no such features are detected in any with a radius of 2 is 4.43 ± 0.11 keV and the temperature of XMM-Newton mosaics of empty fields in observations of sim- southern core of A 223 extracted within the same radius is 5.31± ilar but also much larger depths. We do not detect the bridge 0.10 keV. However, we focus here on the bridge connecting the between the clusters in the 2.0–7.5 keV image. two clusters. L32 N. Werner et al.: Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223

AsshowninFig.2, the spectrum extracted from the filament length of the filament along the line-of-sight, which corresponds shows, between ∼0.5 and ∼1.0 keV, ∼30% emission in excess of to 150 times the mean baryon density of the Universe. From this the background model, which was determined outside the cluster density, assuming l = 15 Mpc, and the temperature of kT = virial radii (see Table 1). We fit this excess emission with a ther- 0.91 keV, we obtain an entropy s = kT/n2/3 ∼ 870 keV cm2. mal model with a temperature of kT = 0.91 ± 0.24 ± 0.07 keV, For a filament with a cylindrical shape, which is 1.2 Mpc wide where the first error is the statistical error and the second error as indicated by the images, and assuming a constant density, we 13 1/2 is due to the uncertainty in the background level, which was de- obtain a baryon mass of 1.8 × 10 (l/15 Mpc) M. Assuming termined by varying the background components in the spectral a baryon fraction of 0.16, the total mass of the filament is ≈ 14 fit within their 1σ errors. The best-fit emission measure of the 1.1 × 10 M. = filament, which is defined to be EM nH nedV,wherenH and The inferred properties of the gas in the filament are remark- ne are the proton and electron number densities and V is the vol- ably consistent with the results of the simulations by Dolag et al. 65 −3 ume of the emitting region, is (1.72 ± 0.50 ± 0.45) × 10 cm . (2006). They predict that within a distance of ≈15 Mpc from Assuming that both errors have a Gaussian distribution, the a massive cluster, filaments connecting clusters have a baryon emission measure of the plasma in the filament within our ex- over-density of ρ/ ρ > 100 inside a radius of ≈1 Mpc from 65 −3 C traction region is EM = (1.72 ± 0.67) × 10 cm (2.6σ). the centre of the filament. They also found that the temperature Fitting individually EPIC/MOS and EPIC/pn the best-fit tem- of the gas along the bridges connecting some nearby clusters can peratures are 0.94 ± 0.23 keV and 0.85 ± 0.30 keV, respectively, be around 107 K, which is the temperature of the gas that we see and the statistical significance of the excess emission is 3.5σ between A 222 and A 223. The predicted X-ray surface bright- and 1.8σ. The derived emission measure depends on the as- ness of filaments according to Dolag et al. (2006)isatmost sumed metallicity. For a higher metallicity, we obtain a lower 10−16 erg s−1 cm−2 arcmin−2. However, this predicted surface 65 −3 emission measure (for Z = 0.3Solar,EM = 1.3 × 10 cm ), brightness was derived for zero metallicity (for a metallicity of while for a lower metallicity the emission measure is higher (for Z = 0.2 Solar, the predicted X-ray flux would increase, due to 65 −3 Z = 0.1Solar,EM = 2.4 × 10 cm and for Z = 0Solar, line emission, by a factor of ∼1.6) and for the case of looking at 65 −3 EM = 4.13 × 10 cm ). The plasma temperature does not the filament edge on. Line emission and a favourable geometry change significantly as a function of the assumed metallicity. could increase the surface brightness to our observed value of The 0.3–10.0 keV luminosity of the filament, within our ex- 10−15 erg s−1 cm−2 arcmin−2. traction region, is 1.4 × 1042 erg s−1.IntheCOSMOSsurvey, If the observed redshift difference is entirely due to peculiar a number of groups have been detected at these luminosities and −1 velocities, ∆z = 0.005 would correspond to ∆v = 1500 km s , redshifts, but their typical spatial extent (r ∼ 1.3 , Finoguenov r 500 and both clusters would be at the same distance, in which case et al., in prep.) is much smaller than that of this filament. the virial radii of the clusters would partially overlap and they By determining the background properties using the two ex- would be at the onset of a merger. However, if this is the case, traction regions at the central chip of EPIC/MOS and fitting only then we would not expect to observe such a large amount of the EPIC/MOS data, we obtain a best-fit emission measure and − gas at these low temperatures between the clusters. Most of the temperature for the filament of EM = (1.7 ± 0.6) × 1065 cm 3 mass from a previously present filament would have been ac- and kT = 0.80±0.19 keV. These values are consistent with those creted by the clusters and the gas in the interaction region would obtained using a background region located at a larger distance have been compressed and shock-heated to temperatures signif- on a different part of the detector. This minimizes chip-to-chip icantly higher than those observed. We therefore conclude that variations in the instrumental noise and allows to include some the redshift difference cannot be attributed entirely to peculiar residual cluster emission in the form of an increased best-fit flux velocities. of the power-law model component. However, part of this increased power-law flux could be due to emission from the fila- The temperature and average density of the observed gas as- ment in the background regions, resulting in an oversubtraction sociated with the filament indicates that we are detecting the of the filament. hottest and densest phase of the WHIM. It is only detectable because of the favourable geometry of the massive filament connecting two large clusters of galaxies. According to simulations 4. Discussion (e.g. Davé et al. 2001), however, the dominant fraction of the WHIM resides in a lower temperature, lower density phase, and We detect X-ray emission from a bridge connecting the clusters its existence still remains to be proven observationally. The de- A 222 and A 223. The temperature of the gas associated with this bridge is kT = 0.91±0.25 keV. If this gas is the intra-cluster tection of the dominant fraction of the WHIM will be possible only using dedicated future instrumentation (e.g. see Paerels medium (ICM) at the cluster outskirts, then assuming an emit- et al. 2008). ting volume with a line-of-sight depth of 2.5 Mpc, our best-fit emission measure corresponds to a density of ∼1×10−4 cm−3 and To further investigate the physical properties of the densest the entropy of the gas is s = kT/n2/3 ∼ 420 keV cm2. Because and hottest gas phase permeating the cosmic web, the observed of heating by accretion shocks, the cluster entropies rise towards sample of cluster pairs for which the large-scale structure fila- 2 the outskirts to values higher than 1000 keV cm outside 0.5 R200 ments can be observed along our line-of-sight needs to be en- (Pratt et al. 2006), and the entropy is expected to rise further out larged by other promising systems. to the virial radius. The low entropy of the bridge implies that the emitting gas is in a filament that has not yet been reached by the shock-heating operating on the ICM in the cluster outskirts. If the redshift difference between the clusters is only due to Acknowledgements. A.F. acknowledges support from BMBF/DLR under grant ff ± 50 OR 0207 and MPG. This work is based on observations obtained with the Hubble flow, their line-of-sight distance di erence is 15 XMM-Newton, an ESA science mission with instruments and contributions di- 3 Mpc. Assuming that the observed emission originates in hot rectly funded by ESA member states and the USA (NASA). The Netherlands gas within the filament connecting the clusters, the mean baryon Institute for Space Research (SRON) is supported financially by NWO, the density of the gas is 3.4×10−5(l/15 Mpc)−1/2 cm−3,wherel is the Netherlands Organization for Scientific Research. N. Werner et al.: Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223 L33

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